‘The Unfettered Mind’ by Takuan Sōhō, Translated by William Scott Wilson
17th-century lessons on applying Zen Buddhism to the martial arts, written from a master of the former to a master of the latter.
‘Mindfulness in Plain English’ by Henepola Gunaratana
What it says in the book title: A straightfoward, practical guide to meditation and mindfulness for those wanting to take up such habits.